The invention relates to a cold-rolling method and installation for a metal band, especially suited for an average yearly production, for example ranging between 300,000 and one million tons.
The invention applies in particular to descaling and rolling steel sheets and strips obtained by hot rolling or by thin continuous casting.
It is known that generally, the manufacture of metal products calls, first of all, for the preparation of a raw product by ingot mould casting or continuous casting, hot transformation by forging and/or hot rolling and cold transformation comprising various steps that depend on the nature of the metal, for example ferritic or austenitic steel, and on the product to be manufactured, for instance ordinary steel, stainless steel, alloyed steel.
Usually, the hot prepared product is subject, successively, to a descaling treatment for descaling, to cold rolling until the requested thickness is obtained and, possibly, to finishing treatments that depend on the type of sheet to be produced, for example annealed, galvanised or other surface treatment.
Cold rolling is conducted, normally, in several successive passes, either in two opposite directions on a reversible mill, or on several rolling stands operating in tandem.
Up to recently, the various cold treatments had always been carried out in a discontinuous fashion in different installations, whereas the product is wrapped into a coil at the end of each step in order to be transferred to the next step. These non-continuous methods therefore require several unwinding and winding operations of coils as well as intermediate storing phases generating significant costs, taking into account the necessary handling tools and staff.
Moreover, in reversible rolling, a minimum band length must remain wound on each coil and is therefore not rolled. These parts outside tolerances cannot be marketed and must hence be eliminated.
To remedy these shortcomings, the last few years have seen the development of continuous line manufacturing methods that enable doing away with coil winding at least for certain intermediate steps. In particular, we already know methods in which certain cold transformation operations are carried out continuously on a single line. For example, coupling pickling with cold rolling enables reducing, to a vast extent, the shortcomings stated above.
A coupled line installation of this type comprises, generally, an inlet section containing a device for unwinding, one after the other, coils to be treated, a pickling section for descaling, generally by immersing the band in a chemical pickling liquid, a cold rolling section and an outlet section comprising means for winding the rolled band into a coil.
To perform continuous running of the band, the inlet section comprises linking means, by welding or stapling, of the downstream extremity, in the running direction, of a first coil with the upstream extremity of the following coil. Thus, we obtain a continuous band running successively through the different sections of the line.
In normal operation, a same running speed, for example 400 m/min or even more, is maintained up to the inlet of the mill and increases then in relation to the reduction in thickness.
However, for various reasons, it is necessary to vary considerably the speed of a section with respect to the adjacent sections, respectively upstream and downstream.
For example, during the time necessary to the connection of extremities of two successive coils, the inlet section is stopped, whereas the band must still be running in the pickling section whose stoppage might cause defects on the metal further to an excessive dwelling in the acid used.
Similarly, it is required, in some cases, to stop or, at least, to slow down the running in the rolling mills, for example for maintenance operations. Indeed, the working rolls are worn quite rapidly and must be replaced periodically with new rolls. During the time necessary to replacement, the rolls are held away from the band and, even if the running of the band is not stopped completely, it should be at least slowed down in order to reduce the length of the band that has not been rolled, which then should be eliminated.
On the other hand, after rolling, the band is wound once more in order to form a coil and, when the said band reaches its maximum length, the band must be shorn to allow evacuation of the wound coil. To avoid complete stoppage of the band during shearing, it is advantageous to use so-called xe2x80x98flyingxe2x80x99 shears composed of two blades mounted respectively on two rotating drums, but the speed must, however be reduced.
After shearing, the band must be wound to the end and the complete coil must be evacuated, then the shorn extremity of the following band must be attached to the reeling plant in order to constitute a new coil
To do so, two coilers operating alternately may be used, with a switching system that enables, after shearing, to direct the upstream extremity of the following band immediately to the second coiler for winding the new coil, whereas retraction of the wound coil on the first coiler is performed in hidden time. A so-called carousel coiler comprising two winding mandrels operating alternately can also be used.
These arrangements enable reducing the time necessary to changing the coil, but the shorn band should run forward at low enough speed to enable its extremity to engage on the chuck and to start the winding process.
It seems therefore that even if perfected arrangements enable to reduce the time necessary to certain operating phases during which the running speeds in the different sections of the line must vary independently from one another, it is necessary to place means for accumulating the band between certain sections. Thus, the band can be accumulated at the outlet of a section when the running is stopped or slowed down downstream and, conversely, when the running is stopped or slowed down in a section, it is possible to continue running, downstream, a band length accumulated previously.
Generally, such a coupled line must comprise at least two means of accumulation placed, respectively, at the inlet and at the outlet of the handling section. Thus, before reaching the end of a coil, a certain length of the band will be accumulated, which will continue to run in the handling section for the time necessary to the connection with the extremity of the following coil. Similarly, if the mill has been stopped, for example for replacing the rolls, the band should be run further in the pickling tanks, while accumulating the pickled length at the outlet of the treatment section.
Obviously, other members are necessary such as tensioning devices for traction load adjustments in the different sections or edge shears.
All these members are obviously quite expensive and call for high energy expenses and maintenance costs.
Indeed, to ensure the necessary high running speeds, the control motors of the various pieces of equipment must be very powerful.
Moreover, after usage, the inlet accumulator of the treatment section must be emptied in order to compensate for later speed variations.
Still, these operations must be carried out very rapidly in order to reduce the transition periods and require therefore motors capable of supplying the necessary accelerations.
Besides, in order to maximise the operation of the mill, the said mill must be suited to certain types of products and the other sections of the line, in particular pickling and finishing sections, must be provided accordingly.
Therefore, although such installations are extremely costly, their operating conditions must paradoxically be sufficiently rigid to ensure profitable production with the quality requested.
Because of the investment costs, the energy expenses and the maintenance costs, such coupled installations had been provided so far only for high production levels, ranging for example between 1 and 2 million tons per annum, if not more. Such capacities are, obviously, justified only for certain types of products and, in other cases, it seems more economical to use conventional installations in which the operations are carried out separately and discontinuously. In particular, for average productions, cold rolling is, normally, conducted in a reversible mill, by successive passes in one direction and in the other.
However, as it is not possible to obtain the same advantages as in a coupled line, such as the suppression of intermediate stocks, diminution of staff requirements, reduction in the surface covered or diminution of band lengths outside tolerance, such discontinuous installations are only profitable for special products such as stainless steels, up to 300,000 or 400,000 tons, or in the case of mini factories producing varied sheet qualities, but in limited quantities.
Conversely, there had not been, so far, average capacity installations, for example between 300,000 and one million tons and enabling economic production of sheet metals of all types.
To solve such a problem, the invention concerns a new method for producing metal bands in a coupled line that enables to reduce the investment and operating costs sufficiently to remain profitable even for an average production, between 300,000 and one million tons.
The invention applies therefore to a coupled line installation comprising as usual, an inlet section of the treatment band, a cold rolling section and an outlet section.
According to the invention, the composition of the treatment fluid is determined so that the metal of the band is not attacked during a dwelling period in the treatment section corresponding to a minimum running speed in the order of a few metres per minute, and the rolling conditions are determined so that the requested qualities of the band can be maintained at minimum rolling speed that may be as low as one meter per minute.
Thus, the running speeds, respectively in the treatment section and in the rolling section remain substantially similar throughout the operating phases, and it is possible to considerably reduce the capacity of the accumulators and, consequently, the investment and operating costs.
Indeed, as stated above, the coupled lines used until now have a very large capacity, exceeding for example, 150,000 tons per month and hence work at very high speed.
Moreover, the mill must be able to realise a significant thickness reduction ratio and usually comprises four or five stands operating in tandem. This leads to significant running speed variations in the line.
For example, the average rolling speed of the band may range between 500 and 1,500 m/mn at the outlet of the rolling section whereas the running speed in the pickling section must necessarily remain lower, for example between 100 and 400 m/mn.
Therefore, in the conventional coupled lines, the accumulators must have a very large capacity, in the order of 400 to 600 m for compensating the necessary speed variations between the different sections, and thus constitute extremely cumbersome and complex installations, comprising several running levels of the band between a series of fixed deflecting rolls and a series of mobile rolls that may move longitudinally to vary the lengths of the different levels.
Normally, the production capacity of an installation, at equal width and thickness, is proportional to the running speed and as the necessary stoppage times of the band, at the inlet and at the outlet of the line, are the same, the capacity to confer the accumulators is more or less proportional to the tonnage to be realised. Consequently, for an installation producing 400,000 tons per annum, i.e. approx. 20% of the capacity of the current coupled lines, the capacity of the accumulators should normally be reduced in the same proportion, which leads to a sizing from 80 m to 120 m. Such accumulators are still very cumbersome and expensive and, for this reason notably, it has not appeared possible so far to make a coupled line profitable for an annual production smaller than 1 million tons.
In order to solve this problem, the inventor has deviated from the usual operating conditions while selecting, on the contrary, to conduct the operations so that it should be possible, while maintaining the desired quality of the band, to use simple and cheap equipment enabling to lower the investment, operating and maintenance costs sufficiently so that such a coupled line is profitable even for average capacities.
To this aim, in a particularly advantageous fashion, during all the operating phases, the relative running speeds, respectively in the inlet section, the treatment section, the rolling section and the outlet section, are adjusted so that the speed difference between two successive sections during an operating phase matches the running, in the fastest section, of an additional band length not exceeding a few ten metres.
Thus, whereas in conventional coupled lines, the band accumulators are very large installations with a complex operation, the invention enables using accumulators with reduced capacity, for example a few ten metres, and with a far simpler constitution.
On the other hand, as can be seen in the following detailed description, the other members of the installation can also be simplified while preserving the qualities of the band produced.
In particular, the running speeds may be reduced considerably, even in the rolling section, and it is therefore possible, at the outlet of the said rolling section, to use fixed type shears cutting the band at a speed not exceeding one metre per minute, as well as winding means comprising a single reeling plant.
Besides, thanks to the possibility of maintaining the quality of the band, even at low speed, it is not necessary to reduce the filling time of the accumulators to the maximum. Consequently, the maximum running speeds, respectively in the inlet section and in the rolling section need not exceed by more than 10% the maximum running speed in the treatment section. Under these conditions, the accelerations may be reduced. Thus, smaller and, consequently, cheaper motors can be used.
As a result, the investment and maintenance costs of a coupled line according to the invention are reduced considerably with respect to those of a conventional coupled line, with very high capacity. On the other hand, as rolling can be prolonged up to very low speed, the yield ratio remains acceptable. It is then possible to produce economically cold rolled coils with an average capacity ranging between 300,000 and one million tons.
Thanks to the advantages provided by the coupling, an installation according to the invention remains profitable even for the production of ordinary quality sheets or strips, for instance in low alloy steel, and will have therefore quite a varied production range whereas, until now, the coupled lines had only been used for certain qualities justifying a high production capacity.
It has been noted, on the other hand, that the capacity foreseen for an installation according to the invention corresponded more or less to the capacity of a continuous lining or annealing line. Consequently, it will be possible to combine an installation according to the invention with other facilities placed downstream the rolling section and enabling to subject the band to various finishing treatments, according to the requested quality. Indeed, the operation of a coupled line with average capacity according to the invention exhibits sufficient flexibility to enable incorporation of a lining with annealing equipment, for example, a galvanisation line, to the same line.